A Golgi and horseradish peroxidase study of the sonic motor nucleus of the oyster toadfish.

The sonic motor nucleus (SMN) of the oyster toadfish Opsanus tau, a single midline structure in the occipital spinal cord and caudal medulla, contains large electrically-coupled motoneurons. Although interpretation is complicated by multiyear growth in soma size, neurons in males may be either large (L) or small (S), whereas females have exclusively S neurons. Golgi stains have allowed separation of five neuron variants (rostral, dorsal, stellate, ventral and caudal) which differ in location, soma shape and size, and direction and pattern of dendritic branching. All variants are present in L and S males and in females, and retrograde transport of horseradish peroxidase indicates that all variants are motoneurons. The SMN is organized into three horizontal layers with rostral and dorsal neurons forming a rostrocaudally arrayed network across the dorsal-dorsolateral surface. Stellate cells are found in the middle layer, and ventral cells with laterally directed dendrites that exit the SMN line the inferior surface. Caudal neurons with caudally directed exiting dendrites are arranged in parallel rows in the caudal fifth of the SMN. We suggest that variant differences in dendritic orientation relate to different patterns of innervation by multiple afferents to the SMN and function to maximize contacts between neurons as a means of facilitating synchronization within the nucleus. Sexual dimorphism has been demonstrated to a minor degree: all variants have larger somas in L fish than S fish, but no difference has been found in primary dendrite diameter. Larger somas would potentially support the greater amount of sound production by nesting males who produce a mating boatwhistle call. Equivalent dendrite diameter in females, who are just as likely as males to grunt, an agonistic call, suggests that female Opsanus have a well developed sonic circuitry compared to Porichthys, another toadfish in which females are typically silent.